Hotmelt adhesives are solventless adhesives which, after melting, are capable of thoroughly wetting surfaces of materials and firmly adhering to them after cooling and solidification. They do not undergo any chemical changes during the melting process. Hotmelt adhesives generally consist of a heterogeneous mixture of substances which form the adhesive component, the cohesive component and the additive component. There are many known starting components for the production of hotmelt adhesives, including for example ethylene/vinyl acetate copolymers, styrene copolymers, block polymers, such as polystyrene/butadiene/polystyrene, polystyrene/polyisoprene/polystyrene, polyamides, atactic polypropylene, natural and synthetic waxes, paraffins, fillers, antioxidants, antiagers, etc.
In the production of hotmelt adhesives, the components are melted in heated stirred tanks and then made up in pelleting or granulating machines. Particular difficulties are encountered in the making-up of so-called contact hotmelts, i.e. hotmelt adhesives with permanent tackiness, because they remain tacky at room temperature.
Various techniques have been developed for the making-up of these adhesives; some of them are described in the journal "Seife-Ole-Fette-Wachse" (1976), pages 279-282. It is known that the cooled, particulate hotmelt adhesive can be after treated to prevent the individual particles from sticking together. To this end, the pellets or granules are coated, for example by powdering, spraying or spray-coating. The disadvantage of this approach lies in the introduction of foreign matter into the adhesive so that its quality is generally reduced.
In another known process for making up hotmelt adhesives, silicone-coated paper or cardboard containers are filled with the hot adhesives. The individual blocks of adhesive are prevented from sticking to one another by leaving the paper container on the blocks until they are remelted by the user. The disadvantage of this process lies in the accumulation of non-reusable packaging material.
Another process for making up hotmelt adhesives, particularly contact hotmelts, is known from DE 32 34 065 A1. The exposure of the adhesive to high temperatures both during production and during processing is said to be significantly reduced in this process by extrusion of the adhesive to form a strand of any length and subsequent wrapping of the strand in a film. For processing, the adhesive is melted together with the film. It is possible in this way to wind the strand into a roll of the particular length required without any danger of the strands sticking together under the pressure of the roll in conjunction with the storage temperatures. Polyolefin films, such as polyethylene films, are used as the film. However, films of PVC or other vinyl chloride polymers and polyvinyl alcohol films may also be used, depending on the nature of the hotmelt adhesive to be wrapped. Where the hotmelt adhesives contain polyamides, polyamide films may also be used.
In the known process, the wrapping of the hotmelt adhesives in the film is generally carried out at a temperature below the melting temperature of the film, i.e. at temperatures in the range from 90 to 180.degree. C. and more particularly at temperatures in the range from 95 to 130.degree. C.
In the embodiment disclosed in the document cited above, a hotmelt adhesive is extruded through the nozzle of an extruder at a temperature of 120 to 130.degree. C. to form a strand 20 to 25 mm in diameter. A 50 mm thick tubular film of polyethylene is guided over the nozzle head, being protected against premature heating by the nozzle head by an insulation which may additionally be coolable. After filling with the adhesive, the tubular film is cooled in a water bath. After cooling, the strand is wound into a roll.
A similar process for the production of hotmelt adhesives is described in DE 31 38 222 C1. In this case, too, a strand of a hotmelt adhesive is wrapped in a polyolefin film, the melting temperature being in the range from 130 to 200.degree. C. and preferably in the range from 150 to 180.degree. C. The document in question does not provide further particulars of the wrapping step, particularly in regard to the cooling phase.
The known filling of tubular films with hotmelt adhesive involves difficulties. The films particularly suitable for melting together with the adhesive generally have melting points below the temperature at which the film is filled with the hotmelt adhesive. Insulating the tubular film from the nozzle head is not sufficient on its own. Instead, provision has to be made for effective cooling of the tubular film while it is being filled to ensure that the tubular film does not develop leaks. Due inter alia to the need for effective cooling, another problem arises. In the conventional filling of tubular films with liquids, the hydrostatic pressure of the liquid inside the tube leads to a uniform and round shape. In the present case, however, the adhesive solidifies only a short time after introduction into the tubular film or at least becomes so viscous that the insides of the tubular film stick very easily to one another. In this case, the cross-sectional area of the tubular film at this point is very small, the cooling effect resulting in rapid solidification there. The tubular film becomes blocked and can only be partly filled. Another disadvantage of the known process lies in the relatively narrow diameter (approximately 20 to 25 mm) of the final adhesive strand. Diameters of around 80 mm are desirable for practical purposes. In the production of diameters as large as these, however, the cooling problems mentioned above and the problems caused by sticking of the tube become even greater. Although DE 32 34 065 A1 refers in general terms to the usefulness of strand diameters in the range from 1 to 10 cm, the making-up of strands such as these by this process involves enormous difficulties.